Research is continuing on experimental and theoretical studies of carrier- and channel-mediated transport of ions across lipid bilayer membranes with the broad objective of understanding, at a molecular level, the selectivity and permeation mechanisms of model carriers, model channels and nerve. Experimental and theoretical studies will be carried out for well defined carriers and channels of the type (e.g. linear and cyclic polypeptides) thought to be present in cell membranes. These studies are directed toward ascertaining the essential similarities and differences between carriers and channels in their bioelectrical properties. Additionally, experimental and theoretical studies will be done of the influence on ion selectivity of the molecular structure of the binding site (as well as of the electronic and molecular structure of the bound ion). These studies should extend the basis, already laid by Hille and Armstrong, for using ions as "probes" of the structure of the Na and K ions channels of nerve, since it is possible in artificial systems to study systematically variation in the cavity size and ligand "field strength" of selected carrier molecules and channels. Our approach encompasses the extension of theoretical considerations to include the effects of rate-limiting steps in the formation of the complexes, their rate of crossing the membrane- solution interface, and the effects of the charge of the polar head groups, as well as extensions to channels.